Concerns about pollution resulting from the operation of internal combustion engines have promoted a considerable increase in interest in fuel cell systems that produce electric power for electric motors, particularly where the electric motor is employed to provide propulsion for a vehicle. Such systems are quite complex in a number of respects. For one, the multiple components of the systems are typically required to fit into a relatively small space. Further, the weight of system components when the system is used in a vehicle must be at a minimum so as to maximize fuel efficiency. Additionally, the systems must be capable of operating efficiently, with prompt response, to a variety of loading conditions corresponding to internal combustion engine operating states such as idle, on the one hand and full power on the other as well as intermediate loadings as might be utilized in a vehicle for relatively constant speed cruising.
As is well known, the fuel cells have electrodes, one of which is subjected to gaseous fuel and another of which is subjected to a gaseous oxidant for the fuel. A typical fuel is hydrogen and a typical oxidant is the oxygen contained in air. The fuel may be molecular hydrogen provided by any of a variety of means as, for example, a source of compressed molecular hydrogen, or a hydrogen rich gas produced by catalytic processes from hydrocarbons as, for example, gasoline or diesel fuel or other organic compounds such as alcohols, most notably methanol.
Many fuel cells conventionally employ internal membranes. Such fuel cells are of the so-called PEM type and have proton exchange membranes, also referred to as polymer electrolyte membranes. The membranes, in order to perform properly, must be kept moist and must not be subjected to inordinately high temperatures. For example, in many systems contemplated today, a maximum fuel cell operating temperature is believed to be on the order of 80-85° C. if problems with membranes are to be avoided. Of course, the specific temperature will depend upon the type of material of which the membrane is made as well as other factors.
To provide proper operation of a fuel cell system, it is necessary that means be provided so that the fuel cell membranes be wetted to the proper degree to avoid damage to or shorten the life of membranes as well as to maintain a desired, high degree of efficiency of operation.
It is also highly desirable that the incoming fuel and oxidant streams be at a desired operating temperature of the fuel cell and be at a desired range of relative humidity to maximize membrane efficiency over a wide range of operating conditions of the fuel cell, particularly in applications where loading of the fuel cell varies over a substantial range.
The present invention is directed to meeting these needs.